Wolwo and calf strain
I have been there - a few times - and I suspect, in my case (hoping the example will help), reduced calf muscle strength plus nervous energy on race days (I don't belief I get nervous anymore, but who knows, so I keep an open mind) have been the main contributory factors. Yes we say we run the same pace in racing that we run in training but who monitors that? I suspect that on the occasions of my calf cramps I ran the first 21km much more easily than I intended (and felt good about it!) but that the cramps which followed were set in place long before (during the first half).
My response: listen to good advice and carry them out - so I try to strengthen my calves (calf raising, and dropping, no weights to start with), walking up hill, long slow hilly runs; and I try to lift weights: dead lift, squats and lunges.
My suspicion is the argument about fluids must surely show up in training as well, esp if the claim about same pace running is made. Also as regular runners (you and I?) we do do our fluids, right? BUt this is not to dismiss the advice, no! just to add my own attempts to those so you cover more bases.
Here's hoping! Good luck!
Mark